The present invention relates to a lens evaluating system with using an interferometer.
Conventionally, an interferometer is used to evaluate a lens, for example to examine whether a lens has errors in shape, and/or uneven refractive index distribution inside the lens. Specifically, the interferometer is used to observe interference fringes representing characteristics of a lens with respect to a reference surface. The interferometer is generally provided with an image receiving element such as a CCD (Charge Coupled Device). Further, the interferometer is generally connected to a data processing device which is used to process image data output by the image receiving element, and then the interference fringe data is displayed on a displaying device connected to the data processing device. An engineer observes the displayed image of the interference fringes, and evaluates the lens.
Recently, a lens which has two different focal lengths (which will be referred to as a dual-focus lens) has been known. Such a lens (i.e., a dual-focus lens) is used, for example, as an optical pick-up device for a DVD system which is capable of reproducing data stored in a DVD and a CD.
When a dual-focus lens is evaluated with use of the conventional evaluating system, however, noise is observed in interference fringe image due to two different focal lengths of the dual-focus lens. An typical example is shown in FIG. 7, in which two types of interference fringes, which correspond to two different focal lengths of the dual-focus lens, are observed as overlapped interference fringes. Note that the noise appears as concentric circular interference fringes in a central part. Such a noise may prevent the observer from correct evaluation of the lens, and an improved evaluating apparatus for a dual-focus lens has been desired.